Dual display liquid crystal display device

ABSTRACT

Disclosed is a dual display LCD device including: a front light unit supplying light; a first polarizer on the front light unit; an LCD panel on the first polarizer; a selective reflection/transmission part on the LCD panel; and a second polarizer on the selective reflection/transmission part; wherein the selective reflection/transmission part selectively reflects the light in a reflection mode to display an image on a front side thereof in a first display mode; and wherein the selective reflection/transmission part passes the light in a transmission mode to display an image on a rear side thereof in a second display mode.

This is a divisional application of application Ser. No. 10/736,591filed Dec. 17, 2003, which claims the benefit of Korean PatentApplication No. 10-2003-0019104 filed on Mar. 27, 2003, which are herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dual display liquid crystal display(LCD) device, and more particularly, to a dual display LCD device inwhich an image can be displayed on both the front side and the rear sideof a single liquid crystal display panel.

2. Description of the Related Art

In general, cathode ray tubes (CRT) have been the most popular displaydevice, but it is inconvenient to use in many situations because of itslarge size and heavy weight in relation to the display area.

Accordingly, a thin flat panel display has been developed that can beinstalled and used anywhere because it is slim in relation to itsdisplay area. Thin flat panel displays are replacing the CRT. Forexample, a thin film transistor liquid crystal display (TFT-LCD) devicehas an improved resolution over other flat panel display devices and aresponse speed as fast as the CRT in displaying moving pictures.

As known to those skilled to the art, LCD devices operation is based onthe optical anisotropy and polarization property of liquid crystalmolecules. Because liquid crystal molecules have a thin and longstructure, it is possible to control the alignment direction of theliquid crystal molecules by artificially applying an electric field.Hence, if the alignment direction of the liquid crystal molecules isarbitrarily controllable, light may be transmitted or screened accordingto the alignment direction of the liquid crystal molecules due to theoptical anisotropy of the liquid crystal so that a color image can bedisplayed.

Active matrix LCD devices have active switching elements with anonlinear characteristic in pixels arranged in a matrix. The activematrix LCD device can display images by controlling the operation ofeach pixel by using the switching elements.

Recently a dual display LCD device capable of displaying an image inboth the front and the rear of the LCD panel is being developed.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a dual display liquidcrystal display (LCD device) and a operating method thereof thatsubstantially obviate one or more problems due to limitations anddisadvantages of the related art.

It is an object of the present invention to provide a dual displayliquid crystal display capable of displaying an image on both the frontside and the rear side of a single liquid crystal display panel, and anoperating method thereof.

It is another object of the present invention to provide a mobilecommunication terminal capable of dually displaying on both the frontside and the rear side of a dual display LCD device employing a singleliquid crystal display panel.

It is a further object of the present invention to provide a selectivereflection and transmission optical device capable of selectivelyreflecting or transmitting an incident light depending upon a linearpolarization state of the incident light.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, adual display liquid crystal display (LCD) device includes: a front lightunit supplying light; a first polarizer on the front light unit; an LCDpanel on the first polarizer; a selective reflection/transmission parton the LCD panel; and a second polarizer on the selectivereflection/transmission part; wherein the selectivereflection/transmission part selectively reflects the light in areflection mode to display an image on a front side thereof in a firstdisplay mode; and wherein the selective reflection/transmission partpasses the light in a transmission mode to display an image on a rearside thereof in a second display mode.

In an aspect of the present invention, there is provided a method foroperating a dual display liquid crystal display (LCD) device. The methodincludes the steps of: converting light from a front light unit, into alinearly polarized light; passing the linearly polarized light throughan LCD panel, and controlling a linear polarization state of the lightpassing through the LCD panel depending on whether a voltage is appliedto the LCD panel; and selectively reflecting/transmitting the light witha selective reflection/transmission part depending on the circularpolarization state of the light that has passed through the LCD panel;wherein the selective reflection/transmission part selectively reflectsthe light in a reflection mode to display an image on a front sidethereof in a first display mode; and wherein the selectivereflection/transmission part passes the light in a transmission mode todisplay an image on a rear side thereof in a second display mode.

In another aspect of the present invention, there is provided a mobilecommunication device comprising: a dual display liquid crystal display(LCD) device including: a front light unit supplying light; a firstpolarizer on the front light unit; an LCD panel on the first polarizer;a selective reflection/transmission part on the LCD panel; and a secondpolarizer on the selective reflection/transmission part; wherein theselective reflection/transmission part selectively reflects the light ina reflection mode to display an image on a front side thereof in a firstdisplay mode; and wherein the selective reflection/transmission partpasses the light in a transmission mode to display an image on a rearside thereof in a second display mode; communication means forcommunicating with an external side; and a controller that controls thedual display LCD device and the communication means and controls onwhich side an image will be displayed.

In a further aspect of the present invention, there is provided aselective reflection/transmission optical device comprising: acholesteric liquid crystal (CLC) film that selectively transmits lighthaving a first circular polarization and reflects light having a secondcircular polarization, wherein the first and second circularpolarizations have opposite directions; a first phase compensation filmat the front of the CLC film that generates a phase difference of aboutλ/4 with respect to the light; and a second phase compensation film atthe rear of the CLC film, for generating a phase difference of λ/4 withrespect to the light; wherein depending on a linear polarization stateof the light that is incident into the first phase compensation film,the selective reflection/transmission optical device passes the linearlypolarized light that has a polarization axis in a first direction, andreflects all the linearly polarized light that has a polarization axiswith an angle of 90° from the first direction.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the present invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of this application, illustrate embodiments of thepresent invention and together with the description serve to explain theprinciple of the present invention. In the drawings:

FIG. 1 is a schematic view illustrating the structure of a dual displayliquid crystal display (LCD device) according to the present invention;

FIG. 2 illustrates an optic axis arrangement of an optical deviceemployed in a dual display LCD device according to the presentinvention;

FIG. 3 is a view illustrating an operation of a dual display LCD devicein case a voltage is not applied to an LCD panel according to thepresent invention;

FIG. 4 is a view illustrating a polarization state of a light propagatedfrom each structural element of a dual display LCD device in case avoltage is not applied to an LCD panel according to the presentinvention;

FIG. 5 is a view illustrating an operation of a dual display LCD devicein case a voltage is applied to an LCD panel according to the presentinvention; and

FIG. 6 is a view illustrating a polarization state of a light propagatedfrom each structural element of a dual display LCD device in case avoltage is applied to an LCD panel according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to a preferred embodiment of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a schematic view illustrating a dual display liquid crystaldisplay (LCD) device according to the present invention. Referring toFIG. 1, the dual display LCD device with a single LCD panel 130 isconfigured to display an image both on the front side and the rear sideof the dual display LCD device. Additionally, the dual display LCDdevice includes a front light unit 110 at the front thereof, and thefront light unit 110 includes a light source 111 at a side thereof. Thefront light unit 110 includes a front surface made of a transparentmaterial and having a transmissivity that passes half of the incidentlight. Accordingly, the light irradiated from the light source 111 maybe reflected from the front surface of the front light unit 110 andpropagated toward the rear of the dual display LCD device. Externallight may also be incident into the LCD panel 130.

The dual display LCD device according to the present invention includesa first polarizer 120 and a second polarizer 170 respectively arrangedon the front surface and the rear surface of the LCD panel 130. A lighttransmission axis of the first polarizer 120 and a light transmissionaxis of the second polarizer 170 are perpendicular to each other (i.e.,at an angle of 90°). As exemplified in FIG. 1, the first polarizer 120at the front side of the LCD panel 130 maybe configured to transmit alinearly polarized light in the Y-axis direction and the secondpolarizer 170 at the rear side of the LCD panel 130 is configured totransmit a linearly polarized light in the X-axis direction.

Additionally, the dual display LCD device according to the presentinvention includes a selective reflection/transmission part 180 thatreflects or transmits the incident light according to the polarizationof the light so as to realize a dual display function using a single LCDpanel 130. The selective reflection/transmission part 180 determines apropagation direction (the front or the rear) for the light, so that theimage display direction is determined. The selectivereflection/transmission part 180, as shown in FIG. 1, includes a firstphase compensation film 140 that phase shifts the incident light by λ/4,a cholesteric liquid crystal (CLC) film 150, and a second phasecompensation film 160 that phase shifts the incident light by λ/4. Forexample, the phase compensation films 140 and 160 may be comprised of aquarter wave plate (QWP).

In addition, the phase compensation films 140 and 160 convert thelinearly polarized incident light into circularly polarized light, andconvert the circularly polarized incident light into the linearlypolarized light. The CLC film 150 may selectively reflect or transmit aright handed circular polarization (RHC) light or a left handed circularpolarization (LHC) light.

The selective reflection/transmission part 180 reflects or transmitslight based upon it circular polarization and its directionalinclination. The elements of the selective reflection/transmission part180 are arranged as shown in FIG. 2. FIG. 2 is a view illustrating anoptical axis arrangement of the selective reflection/transmission part180 in the dual display LCD device according to the present invention.The selective reflection/transmission part 180 includes the first phasecompensation film 140, the CLC film 150, and the second phasecompensation film 160. The second polarizer 170 is behind the secondphase compensation film 160.

The optical axis of the first phase compensation film 140 is at an angleof +45° with respect to the rubbing (RIB) alignment direction (X-axis)of the liquid crystal within the LCD panel 130, and the optical axis ofthe second phase compensation film 160 is at an angle of −45 withrespect to the X-axis. Additionally, the transmission axis of the secondpolarizer 170 is aligned with the X-axis.

Accordingly, the selective reflection/transmission part 180 functions asfollows. In the case when the linearly polarized light having apolarization in the first direction is incident on the first phasecompensation film 140, the selective reflection/transmission part 180transmits all the linearly polarized light having the first directionthrough the first phase compensation film 140. Further, in the case whenthe linearly polarized light having a polarization at an angle of 90°with respect to the first direction is incident on the first phasecompensation film 140, the selective reflection/transmission part 180reflects this linearly polarized light from the first phase compensationfilm 140.

When the linearly polarized light having the first direction is incidenton the first phase compensation film 140, the linearly polarized lighthaving the first direction passes through the CLC film 150 and thesecond phase compensation film 160. At this time, the transmitted lightis linearly polarized light having the first direction.

When the linearly polarized light having a polarization at an angle of90° with respect to the first direction is incident on the first phasecompensation film 140, the linearly polarized light polarized at anangle of 90° to the first direction is all transmitted through the firstphase compensation film 140 and reflected from the CLC film 150, tothereby be again transmitted back through the first phase compensationfilm 140. At this time, the reflected light is the linearly polarizedlight having a polarization at an angle of 90° with respect to the firstdirection.

A description of how incident light reflects or transmits principlethrough the dual display LCD device depending upon the linearpolarization state will be described in greater detail below.

First, the description of operation of the dual display LCD device whena voltage is not applied to the LCD panel 130 will be presented,followed second by the case when a voltage is applied to the dual LCDdevice.

FIG. 3 is a view illustrating the operation of the dual display LCDdevice when no voltage is applied to the LCD panel 130 according to thepresent invention, and FIG. 4 is a view illustrating the polarizationstate of the light propagated from each structural element of the dualdisplay LCD device in accordance with FIG. 3 according to the presentinvention.

Referring to FIGS. 3 and 4, when a voltage is not applied to the LCDpanel 130, the incident light is linearly polarized in the direction ofY-axis while passing through the first polarizer 120 thereby beingconverted into light linearly polarized along the Y-axis

. And, as this linearly polarized light

propagates through the LCD panel 130, the polarization is rotated 90° tobe polarized along the X-axis (⊙).

Thus, as the light linearly polarized along the X-axis (⊙) travelsthrough the first phase compensation film 140, the light is convertedinto right handed circularly polarized (RHC) light. The RHC light passesthrough the CLC film 150. This can be achieved through a CLC film 150that reflects LHC light and transmits RHC light.

Accordingly, the RHC light passes through the second phase compensationfilm 160 thereby being converted into light linearly polarized along theX-axis (⊙). Thus, this light (⊙) passes through the second polarizer 170with a polarization axis along the X-axis. Therefore when a voltage isnot applied to the LCD panel 130, the incident light all passes throughthe rear of the dual display LCD device, and the dual display LCD deviceoperates in a transmission mode thereby allowing the rear of the dualdisplay LCD device to be in a ‘white’ state. Additionally, the incidentlight is not reflected at the front surface of the dual display LCDdevice thereby allowing the front of the dual display LCD device to bein a ‘black’ state. As such, the dual display LCD device using a singleLCD panel may be embodied.

FIG. 5 is a view illustrating the operation of the dual display LCDdevice when a voltage is applied to the LCD panel 130 according to thepresent invention, and FIG. 6 is a view illustrating the polarizationstate of the light propagated from each structural element of the dualdisplay LCD device in accordance with FIG. 5 according to the presentinvention.

Referring to FIGS. 5 and 6, when a voltage is applied to the LCD panel130, the incident light is linearly polarized in the direction of Y-axiswhile passing through the first polarizer 120 thereby being convertedinto the light linearly polarized along the Y-axis

, and the polarization of the linearly polarized light

does not change as it passes through the LCD panel 130. As this lightlinearly polarized along the

the Y-axis passes through the first phase compensation film 140, thelight is converted into LHC light. Next, the LHC light is reflected fromthe CLC film 150. This can be achieved through a CLC film 150 thatreflects LHC light and transmits RHC light. The incident LHC light isreflected from the CLC film 150 toward the front of the dual display LCDdevice.

The LHC light reflected from the CLC film 150 passes back through thefirst phase compensation film 140. The LHC light passing through thefirst phase compensation film 140 has a polarization axis at an angle of135° with respect to the X-axis. Accordingly, the reflected light isconverted into light linearly polarized along the Y-axis

so that is passes through the first phase compensation film 140.

The propagation characteristic of the light may be analyzed using theJones' matrix as follows. When light linearly polarized along the Y-axis

passes through the first phase compensation film 140 thereby beingconverted into the LHC light, and then the converted LHC light isreflected from the CLC film 150 back through the first phasecompensation film 140 thereby being converted back into the lightlinearly polarized along the Y-axis

. The propagation characteristic of the linearly polarized light

can be expressed as describe below. The light linearly polarized alongthe Y-axis

may be obtained from the light passing through the first phasecompensation film 140 and being incident on the LCD panel 130, bycalculating the following expressions:

$\begin{pmatrix}{\cos \; 3{\pi/4}} & {{- \sin}\; 3{\pi/4}} \\{\sin \; 3{\pi/4}} & {\cos \; 3{\pi/4}}\end{pmatrix}$ $\begin{pmatrix}^{{- {\pi}}/2} & 0 \\0 & ^{{\pi}/2}\end{pmatrix}$ $\begin{pmatrix}{\cos \; 3{\pi/4}} & {\sin \; 3{\pi/4}} \\{{- \sin}\; 3{\pi/4}} & {\cos \; 3{\pi/4}}\end{pmatrix}$ ${\begin{pmatrix}{\cos \; {\pi/4}} & {{- \sin}\; {\pi/4}} \\{\sin \; {\pi/4}} & {\cos \; {\pi/4}}\end{pmatrix}\begin{pmatrix}^{{- {\pi}}/2} & 0 \\0 & ^{{\pi}/2}\end{pmatrix}\begin{pmatrix}{\cos \; {\pi/4}} & {\sin \; {\pi/4}} \\{{- \sin}\; {\pi/4}} & {\cos \; {\pi/4}}\end{pmatrix}} = \begin{pmatrix}0 \\1\end{pmatrix}$

Thus, the light linearly polarized along the Y-axis

does not change its polarization state while passing through the LCDpanel 130 which is in an optically inactive state, and passes throughthe first polarizer 120 with the Y-axis functioning as the polarizationtransmission axis thereof.

As a result, when a voltage is applied to the LCD panel 130, theincident light is all reflected at the front surface of the dual displayLCD device operated in a reflection mode thereby allowing the front ofthe dual display LCD device to be in a ‘white’ state. Also, the incidentlight is not transmitted through the rear of the dual display LCD deviceoperated in a transmission mode thereby allowing the rear of the dualdisplay LCD device to be in a ‘black’ state. As such, the dual displayLCD device using a single LCD panel may be embodied.

A first substrate with a color filter, a liquid crystal and a secondsubstrate with a TFT array for the LCD panel 130 according to thepresent invention may be sequentially arranged starting from the frontlight unit 110. Also, the first substrate with the TFT array, the liquidcrystal and the second substrate with the color filter for the LCD panel130 may be sequentially arranged starting from the front light unit 110.

At this time, the first substrate and the second substrate may be alsoreplaced with a plastic substrate or a glass substrate having a lowreflection rate. Also, a black matrix provided on the LCD panel 130 usesa material having a low reflection rate for the light propagated fromthe front light unit 110. Additionally, the optical elements of eachpolarizing portion have a low reflection rate.

Meanwhile, because the dual display LCD device configured as describedabove may be used as a dual display element, if the dual display LCDdevice is adapted to a mobile communication terminal (mobile portablecommunication, PDA, etc.), an image may be displayed in both directions,i.e. on the front side and rear side, so that more image displayfunctions may be realized in the mobile communication terminal. Forexample, a mobile communication terminal may include communication meansfor communicating with an external side and a controller for controllingthe communication means and the dual display LCD device and forcontrolling the direction in which an image is displayed on the dualdisplay LCD device.

For example, a folding-type mobile communication device can select adisplay direction for the image depending upon the device being in aclosed state or an open state. Further, a sliding-type mobilecommunication device may set the display direction for the image displayportion.

As described above, the dual display LCD device with a single LCD panelhas the advantages of displaying the image on both the front and therear of the LCD panel, and thereby resulting in a thinner dual displayelement and reduced manufacturing cost.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1-6. (canceled)
 7. A method for operating a dual display liquid crystaldisplay (LCD) device, the method comprising the steps of: convertinglight from a front light unit, into a linearly polarized light; passingthe linearly polarized light through an LCD panel, and controlling alinear polarization state of the light passing through the LCD paneldepending on whether a voltage is applied to the LCD panel; andselectively reflecting/transmitting the light with a selectivereflection/transmission part depending on the circular polarizationstate of the light that has passed through the LCD panel; wherein theselective reflection/transmission part selectively reflects the light ina reflection mode to display an image on a front side thereof in a firstdisplay mode; and wherein the selective reflection/transmission partpasses the light in a transmission mode to display an image on a rearside thereof in a second display mode.
 8. The operating method of claim7, wherein the selective reflection/transmission part comprises: acholesteric liquid crystal (CLC) film that selectively transmits lighthaving a first circular polarization and reflects light having a secondcircular polarization, wherein the first and second circularpolarizations have opposite directions; a first phase compensation filmat the front of the CLC film that generates a phase difference of aboutλ/4 with respect to the light; and a second phase compensation film atthe rear of the CLC film, for generating a phase difference of λ/4 withrespect to the light.
 9. The operating method of claim 7, wherein thefirst phase compensation film and the second phase compensation film arecomposed of a quarter wave plate (QWP).
 10. The operating method ofclaim 7, wherein an angle between the optical axes of the first phasecompensation film and the second phase compensation film is about 90°.11-12. (canceled)
 13. A selective reflection/transmission optical devicecomprising: a cholesteric liquid crystal (CLC) film that selectivelytransmits light having a first circular polarization and reflects lighthaving a second circular polarization, wherein the first and secondcircular polarizations have opposite directions; a first phasecompensation film at the front of the CLC film that generates a phasedifference of about λ/4 with respect to the light; and a second phasecompensation film at the rear of the CLC film, for generating a phasedifference of λ/4 with respect to the light; wherein depending on alinear polarization state of the light that is incident into the firstphase compensation film, the selective reflection/transmission opticaldevice passes the linearly polarized light that has a polarization axisin a first direction, and reflects all the linearly polarized light thathas a polarization axis with an angle of 90° from the first direction.14. The selective reflection/transmission optical device of claim 13,wherein the first phase compensation film and the second phasecompensation film are composed of a quarter wave plate (QWP).
 15. Theselective reflection/transmission optical device of claim 13, whereinthe first phase compensation film and the second phase compensation filmare arranged to have optical axes at an angle of 90° to one another. 16.The selective reflection/transmission optical device of claim 13,wherein when the light incident into the first phase compensation filmis linearly polarized in the first direction and the light passesthrough the first phase compensation film.
 17. The selectivereflection/transmission optical device of claim 13, wherein when thelight incident into the first phase compensation film is linearlypolarized light having an angle of 90° with respect to the firstdirection and the light reflects from the first phase compensation film.